The xylenes, para-xylene, meta-xylene and ortho-xylene, are important intermediates which find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid which is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production. See, for instance, Robert A. Meyers, HANDBOOK OF PETROLEUM REFINING PROCESSES, Second Edition, McGraw-Hill, 1997, Part 2, for a discussion regarding making xylenes.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate. Adjustment of isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Isomerization converts a non-equilibrium mixture of the xylene isomers which is lean in the desired xylene isomer to a mixture which approaches equilibrium concentrations. The isomerization results in by-products such as benzene, toluene and C9+ aromatics being co-produced. In typical processes, the isomerate is distilled in a deheptanizer column to provide an overhead containing benzene and toluene and a bottoms stream containing para-xylene and other C8 aromatics as well as C9+ aromatics. The bottoms stream is passed to a xylene distillation column for separation of the C8 aromatics from the C9+ aromatics. The C8 aromatics are passed to the xylene isomer recovery.
The feed to the xylene-isomer recovery must be highly free of C9+ aromatics in order to meet product quality. Currently, the feed can contain no more than about 500 ppm-mass (mass parts per million) C9+ hydrocarbons, and preferably less than about 100 ppm-mass 1,4-methylethylbenzene. Usually the xylene-containing feeds as well as the recycle from the deheptanizer column contain significant amounts of C9+ hydrocarbons, e.g., often in excess of 2000, or even 3000, ppm-mass. Accordingly, a xylene column is used to separate these C9+ hydrocarbons from the C8 aromatics stream to be fed to the xylene-isomer recovery operation. Due to the proximity of the boiling points of C8 aromatics and C9 aromatics such as 1,4-methylethylbenzene, the xylene column can be quite large, often with over 90 theoretical plates, and involve a high reflux to feed ratio and thus substantial reboiler energy consumption.
Due to the large scale of commercial facilities to produce xylenes, especially para-xylene, even small improvements in capacity, capital costs or variable costs, such as reboiler energy consumption, can represent a material economic benefit to the xylene-isomer producer.